Various types of electrostatic copying systems are in use today, including laser-beam printers which can produce copies of documents through the use of a computer. Dry development is performed in these systems by either a single component process using only a toner, or dual component system using a toner and a carrier. Iron oxides are frequently used in both toners and carriers.
Magnetic particles for use as toners and carriers are required to have certain characteristics. Particularly in single component toners they are required to have a sufficiently high magnetic saturation so as to form a good magnetic brush in the machine, while having low remanent magnetization and coercivity to avoid magnetic agglomeration of the particles. In fine particle toners, the iron oxide is required to also have a small particle size so as to evenly distribute the magnetic particles through the toner particles. However, coercivity and remanent magnetization usually increase as the diameter of the particle is decreased, as long as the superparamagnetic limit is not exceeded, so the preparation of a very small particles of low coercivity and low remanent magnetization represent conflicting desirable properties of magnetic materials.
Phosphorus compounds are used extensively in the art of magnetic iron oxide production, but have rarely been used in spherical or polyhedral iron oxides. In the preparation of acicular iron oxides, phosphorus is sometimes used in the alpha-FeOOH stage to improve particle size distribution (Japanese Patent Public Disclosure No. 25202/1983 and Japanese Patent Publication No. 18766/1993) and has been used to retard crystal growth in FeOOH (Japanese Patent Publication 25546/1964). U.S. Pat. No. 3,652,334 teaches the use of phosphorus as an antisintering agent in the thermal conversion of FeOOH to Fe.sub.2 O.sub.3 and/or Fe.sub.3 O.sub.4. However, all of these examples are directed toward ensuring that the iron oxide particles produced will have improved dispersability or improved shape anisotropy by virtue of retaining their acicularity. European Patent 832,848 describes the use of phosphorus and aluminum together to reduce the magnetic remanence of polyhedral magnetites, but does not in this instance describe the reduction in particle size to produce a very high specific surface area product. Japanese Patent Public Disclosure No. 169717/1996 teaches the use of phosphorus to produce a low remanence magnetite with good polymer dispersability, but does not mention reduction of particle size. Japanese Patent Publication No. 46526/1985 teaches the use of phosphorus in the formation of magnetite particles, but this method is used to produce cobalt containing magnetites of high coercivity and remanence by adding P and Co together.